This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Ubiquitination is among the most conserved of molecular modifications. Attachment of the 8.6- kDa protein can take many forms and serve as many distinct signals for protein fate. The covalent attachment of Ub to a substrate proceeds via a multi-enzyme process centered around an E2 ubiquitin conjugating enzyme. Though poorly understood, the E2~Ub conjugate appears to influence E3 ligase, substrate, and linkage type specificities. This proposal approaches this issue by analyzing differences in structural dynamics among E2~Ub conjugates. NMR analysis has revealed flexibility in the E2~Ub conjugate. Additionally, the degree and pattern of interdomain movements appears to be largely E2 dependent. SAXS is a powerful alternative to NMR for studying proteins in solution. Rapid access to SAXS is required to corroborate this hypothesis for impending publication.